[bobcowan]

Generally speaking, you need to go down about 2 jet sizes. That varies by engine build, though.

Sometimes, backfiring is caused by a lean mixture. So you might need to go down 1 or 0 jet sizes.

Check all the other issues that can cause a back fire first. Once those are fixed, then starting adjusting mixture.

Don't guess. Spend the money on a sensor and actually measure mixture before making changes. Otherwise, it's very easy to go from bad to worse.

ITB are extremely sensitive to balance. If the balance is not perfect, no amount of tuning will make it run well. Buy a synchronometer.